Cementation is the process of deposition of dissolved mineral components in the interstices of porous materials followed by the sticking together of material to form a cemented material. In nature, cementation is a geological process by which sedimentary rocks such as sandstone and limestone are formed. Sandstone and limestone are formed primarily through the precipitation of calcite cement.
Calcite is the least soluble and most stable polymorph of calcium carbonate. Other polymorphs of calcium carbonate are aragonite and vaterite. Supersaturated solutions of calcium carbonate precipitate as unstable amorphous calcium carbonate which is spontaneously converted to the metastable polymorph vaterite. Conversion of vaterite to calcite occurs rapidly in the presence of water, with almost all vaterite converting to calcite within a period of 24 hours. In soil, calcite bridges adjacent soil particles, cementing soil grains together to form a cemented sand or sandstone. In addition to cementing soil grains together, fine particulate CaCO3 can reduce the pore space in the soil matrix and increase the soil's ability to resist shear.
Many naturally occurring microorganisms present in soils, especially bacteria of the family Bacillacae such as bacteria of the genera Bacillus, Sporosarcina, Sporolactobacillus, Clostridium and Desulfotomaculum are able to hydrolyze urea (NH2)2CO in the presence of water to ammonium and carbonate ions by the following reaction.(NH2)2CO+2H2O→2NH4++CO32−
The generation of NH4+ ions increases local pH and, in the presence of calcium ions and the availability of nucleation sites, the carbonate ions react spontaneously with the calcium ions to form calcium carbonate by the following reaction.Ca2++CO32−→CaCO3 
Because calcium carbonate will spontaneously form the stable polymorph calcite which cements soil grains together, microbial cementation has been conceived to be useful in many applications. Kucharski, U.S. patent application Ser. No. 2008/0245272 discloses that microbial biocementation can be used for civil engineering applications such as for fabricating, stabilizing, and reinforcing retaining walls, embankments, and tunnels of for stabilizing sands in earthquake zones at risk of liquefaction, for mining applications such as to provide support for ground that is broken during mining, to strengthen tailing dams to prevent erosion, and to bind dust particles, for construction applications such as to create “instant” pavements such as roads and runways, for restoration applications such as to preserve, restore, and strengthen weathered mortar and masonry, for environmental applications such as the stabilization and removal of pollutants such as heavy metals, fibers, and radioactive elements, and the control of erosion, and for other uses such as the creation of filters including the immobilization of bacteria into a cemented biofilter.
Kucharski discloses a method for forming a cement by combining a starting material such as rock, sand, soil, or clay with particular amounts of (1) an exogenous microorganism that is capable of producing urease, (2) urea, and (3) calcium ions. The method of Kucharski, however, presents several disadvantages, some of which are dealt with in the Kucharski application itself.
One disadvantage is that, due to the fact that the three above components are injected into the starting material and that calcite formation rapidly occurs in such circumstances, calcite formation and resultant cementation tends to concentrate at or near the point of injection of the three components into the starting material. This problem is discussed in Whiffin et al,
“Microbial Carbonate Precipitation as a Soil Improvement Technique,” Geomicrobiology Journal, 24:417-423 (2007). Whiffin, also one of the co-inventors in the Kucharski application, discloses that injection of bacteria together with reagents can result in clogging of the system near the injection point due to the rapid production of calcite. Whiffin discloses a process by which bacteria and reagents may be injected into a starting material without clogging of the material. However, in FIG. 5 on page 420, Whiffin shows that, although an average column volume of 59.2 kg CaCO3/m3 was obtained, the distribution of calcium carbonate in the column was very uneven with values of more than twice the average occurring at a distance of 100 cm from the injection point. Thus, even though clogging did not occur, the method of Whiffin results in an uneven distribution of calcite throughout a starting material and thus in uneven strengthening throughout the starting material.
A second disadvantage of the method of Kucharski is that of loss of microorganisms following their addition to the starting material, discussed in Kucharski in paragraph 0072. Kucharski discloses that this problem may be dealt with by fixing the microorganisms in the starting material prior to cementation. Thus, as stated in paragraph 0072, the method of Kucharski involves the sequential steps of (1) applying the microorganism to the starting material, (2) fixing the microorganism in the starting material, and (3) then combining the starting material incorporating the fixed microorganisms with urea and calcium ions.
A third disadvantage of Kucharski is that, by necessity, the method of Kucharski utilizes a finite number of microbial species, and typically utilizes only one microbial species. Natural environments such as soil contain numerous microbial species that exist in a complex ecological framework. Injection of a monoculture or of a culture containing several microbial species disrupts the ecological framework that existed prior to injection. Moreover, microbes that are injected into a starting material are at a competitive disadvantage to organisms already present that are adapted to the local environment, resulting in loss of a large portion of the injection microbes.
Additionally, because different microbial organisms are able to produce calcite at different optimum values of pH, pH and other environmental parameters must be optimized depending on the particular microorganism that is injected into the starting material.
These disadvantages of Kucharski render the method disclosed therein unpractical for many applications. A significant need exists, therefore, for a method of microbial biocementation that overcomes the disadvantages of the prior art.